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Árnadóttir, Líney

Publications and source records attributed to Árnadóttir, Líney.

5 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Dynamic Activation of Single-Atom Catalysts by Reaction Intermediates: Conversion of Formic Acid on Rh/Fe 3 O 4 (001)

The stability and activity of supported single-atom catalysts (SACs) represent critical yet opposing factors limiting our ability to explore and exploit their unique properties. Here, this study demonstrates the operation of a switchable catalyst that is activated in the presence of surface intermediates and reverts back to a stable but inactive form when the reaction is completed. We employ atomically defined Rh-Fe 3 O 4 (001) catalysts to demonstrate how structurally stable Rh, bound in surface octahedral Fe sites, gets destabilized to form highly active Rh adatoms and small clusters. Conversion of formic acid, leading initially to surface formate and hydroxyl species, is employed as a model reaction to probe the dynamics of such processes. We find that surface hydroxyl recombination to water through the Mars van Krevelen mechanism reduces Rh coordination, triggering its conversion to active Rh adatoms. Since such lattice oxygen exchange is observed in many acid-base and redox chemistries, the process can be broadly applicable to controlling the activation of the range of SACs.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Metal Doping Regulates Electrocatalysts Restructuring During Oxygen Evolution Reaction

High-efficiency and low-cost catalysts for oxygen evolution reaction (OER) are critical for electrochemical water splitting to generate hydrogen, which is a clean fuel for sustainable energy conversion and storage. Among the emerging OER catalysts, transition metal dichalcogenides have exhibited superior activity compared to commercial standards such as RuO 2 , but inferior stability due to uncontrolled restructuring with OER. Here, in this study, we create bimetallic sulfide catalysts by adapting the atomic ratio of Ni and Co in Co x Ni 1-x S y electrocatalysts to investigate the intricate restructuring processes. Surface-sensitive X-ray photoelectron spectroscopy and bulk-sensitive X-ray absorption spectroscopy confirmed the favorable restructuring of transition metal sulfide material following OER processes. Our results indicate that a small amount of Ni substitution can reshape the Co local electronic structure, which regulates the restructuring process to optimize the balance between OER activity and stability. This work represents a significant advancement in the development of efficient and noble metal-free OER electrocatalysts through a doping-regulated restructuring approach.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

The role of nonmetallic ion substitution in perovskite LaCoO 3 for improved oxygen evolution reaction activity

Transition metal perovskite (ABO 3 ) is an emerging type of oxygen evolution reaction (OER) electrocatalyst that shows reasonably good activity and moderate stability. Although efforts have been made to improve perovskite’ OER performance by various element substitution at A/B-site, the influence of ion, particularly non-metallic ion, substitutions on the OER mechanism are rarely studied. More and more evidence has shown that the metal-center theory has failed to explain lots of OER-related phenomena. Therefore, it is urgent to understand how the cation and anion sites in perovskite determine OER performance. In this work, we used a Fe and P co-doped LaCoO 3 as a model system to explore the influence of substitution in perovskite by combining operando/ex-situ X-ray characterization and density functional theory (DFT). We observed enhanced OER catalytic activities in co-doped materials, which are attributed to the stronger transition-metal-oxygen-bonding-covalency (TMOBC). The detailed analyses by O K-edge XAS, electrochemical performance, and DFT suggest that the hybridization between O 2p and transition metal 3d e g orbitals could be a more credible descriptor of perovskite for OER, which is the combination of e g orbital theory and TMOBC theory. The finding in our work provides insights into the OER catalysis mechanism on metal oxides, which could guide new design of cost-effective oxide electrocatalysts.

25 ENERGY STORAGE↗
DOE OSTI2022

Unraveling the role of tungsten as a minor alloying element in the oxidation NiCr alloys

Ni-based superalloys offer a unique combination of mechanical properties, corrosion resistance and high temperature performance. Near ambient pressure X-ray photoelectron spectroscopy was used to study in operando the initial steps of oxidation for Ni-5Cr, Ni-15Cr, Ni-30Cr and Ni-15Cr-6W at 500 °C, p(O 2 )=10 -6 mbar. The comparison of oxide evolution for these alloys quantifies the outsized impact of W in promoting chromia formation. For the binary alloys an increase in chromia due to Cr-surface enrichment is followed by NiO nucleation and growth thus seeding a dual-layer structure. The addition of W (Ni-15Cr-6W) shifts the reaction pathways towards chromia thus enhancing oxide quality. Density functional theory calculations confirm that W atoms adjacent to Cr create highly favorable oxygen adsorption sites. The addition of W supercharges the reactivity of Cr with oxygen essentially funneling oxygen atoms into Cr sites. The experimental results are discussed in the context of surface composition, chemistry, reactant fluxes, and microstructure.

36 MATERIALS SCIENCE↗